Body Temperature and Thermoregulation

Learning Objectives nTo understand the distinction between endothermy and ectothermy, and between homeothermy and poikilothermy nTo be familiar with the contribution of metabolic rate, surface insulation and circulatory adaptations to the maintenance of body temperature in endotherms nTo understand how some insects and mammals use endothermy on a temporary basis

Before we begin: Definitions 1.Importance of metabolism Endotherm - body temperature depends on heat production by animal metabolism Ectotherm - body temperature depends on heat acquired from the environment, and is little influenced by metabolic rate 2.Importance of Body Temperature Homeotherm - body temperature constant (usually as a result of endothermy) Poikilotherm - body temperature variable and strongly influenced by environment

Why keep warm ? nsurface / volume ratio of large animals nmore “efficient”

Recall: MR increases with body weight, but less than proportionately log MR log body weight a 0.75 = b logMR = a + b. log bwt MR = a. bwt b a

Animalab body temp ( o C) Mammals Birds Lizards Fish Crabs Lizards at 38 o C Relationship between metabolic rate and body weight for animals maintained at 20 o C Birds and mammals Other animals

Maximum speed Speed of passage of food through gut Lizards and fish better of at warmer temperatures

Mammals and birds are ‘gas guzzlers’ Exploit elevated MR to maintain stable temperature =>Very costly (food, resources) =>Generates activity that is independent of external conditions Birds and mammals Other animals

Why aren’t lizards as efficient as birds or mammals? a.Body temperature - high and maintained in birds and mammals b.Intrinsically high metabolic rate: if lizard placed at 38 o C MR will increase, but not to same level as in birds and mammals  Physiology of birds and mammals ‘wired up’ to run faster than other animals Observed at level of O 2 consumption Observed in organ system organisation: lungs, gut absorptive surface, design of circulatory system

Metabolic rate of cardinals reared at different temp too hot below here need to use extra energy to keep warm temperature, °C oxygen consumption

Summary so far nA large part of basal metabolism is to keep warm Now onto: how do we minimise heat loss?

Surface insulation: a barrier to loss of metabolically generated heat thermal conductivity (W m -2 o C -1 ) Water0.61 Air0.025 Muscle0.48 Fur0.04 Blubber 0.24 (subcutaneous fat)[smaller numbers are better] Heat lost through water more rapidly than through air. Muscles are poor at retaining heat, but subcutaneous fat is better Fur has low conductivity: terrestrial mammals can maintain a 30 o C gradient between skin surface and external temperature (i.e. trapping air of low conductivity)

Blubber has 6x conductivity of fur: i.e. needs 6x thickness of fur to generate same insulation. body 38 o C external fur Temp at skin surface 38 o C body 38 o C blubber external Temp at skin surface = external temp seal v dog…

Seal cross-section

Blubber as surface insulation in seals Skin surface in water = water temp Skin surface in air > air temp Seals must lose heat to air to avoid overheating

Keeping extremities warm? But what about the fins? countercurrent heat exchanger!

Retaining and maintaining heat: the counter current heat exchanger

Countercurrent ndolphin flippers nhens’ legs n Hen’s feet 2 mm

Metabolic rate declines as temperature declines, but down to 0 o C, no heat loss from feet. When temperature below freezing, vasodilators open to prevent feet from freezing

Most fish, water temp = body temp, as loss of heat through gills. In tuna: sustained fast swimming requires temp of ~30 o C Keeping tuna muscle warm Achieved by counter current blood flow

tuna heat exchanger v v v a a a 0.1 mm

Summary so far nA large part of basal metabolism is to keep warm nKeeping warm u insulation F fur, blubber u countercurrent heat exchangers F extremities F regional temperature control Now onto: avoiding overheating

Seals pump blood to body surface in air to achieve cooling via vasodilator. i.e. pattern of blood flow regulates heat loss Seal heat loss

Vasodilation njackrabbit ears before and after exercise all less than 10°C above 30°C air temp 6°C

guanaco heat losses nrate of heat loss = 1/fur length

Heat loss nActually, major heat loss is by evaporation u sweat u panting nwhy: evaporation uses a lot of energy: u 418 J to heat water from 0 to 100 °C u but 2443 J to boil it [1 g]

Carotid artery in ungulates: when blood temp rises (during a chase to 44°) danger to brain: venous blood cooled in rete and nasal cavity (=<40°C) Heat loss in a hurry

Heat loss in the desert nman (70 kg): u BMR needs 0.12 l / hr evaporation u heat from sun = 1.2 l / hr ncamel (400 kg): u temp goes from 34 to 41 °C (≈2900kcal, 5 l water) F store heat until night F reduce difference between outside + camel u evaporation 0.9 l /hr u fur reduces heat inflow (shearing doubles evaporation)

Summary so far nA large part of basal metabolism is to keep warm nKeeping warm u insulation F fur, blubber u countercurrent heat exchangers nHeat loss by evaporation Now onto: Facultative endotherms

Facultative endotherms If there are so many advantages to endothermy, why aren’t all species endotherms? a) costly on resources, especially food to maintain high metabolic rate b) costly for small animals with high specific metabolic rates Small animals can gain the best of both worlds by employing endothermy only when needed (facultative)

Facultative endothermy in Insects: Early season bumble bees Night moths Achieved by synchronized muscle activity (with no movement)

countercurrent

co-contraction up/down alternate

Summary so far nA large part of basal metabolism is to keep warm nKeeping warm u insulation F fur, blubber u countercurrent heat exchangers nHeat loss by evaporation nEndothermy u Facultative in insects Now onto: Torpor

Torpor: some birds and mammals exhibit torpor/adaptive Hypothermia Reduces the metabolic rate in response to: low external temperature Andlow food availability Torpor is under physiological control

Characteristics of Torpor: Reduced metabolic rate, but maintenance of control (avoids freezing, i.e. during hibernation) Reduced motor and sensory function, more comatose than Sleeping (low heart rate, low respiratory rate) Can display arousal and return to ‘normal’ body temperature metabolic rate or endogenous heat production

Characteristics of Torpor (cont): Generally small animals, small mammals, birds, rodents hummingbirds: due to? Energetic cost of maintaining high body temp for small animals? Costs of arousal, costly for large animals However: bears in winter dormancy: reduce MR by 50%, body temp by 5 o C

Torpor in birds Eulampis

Torpor in bats

Torpor in mammals (marmot) use fat rather than glucose

energy saved Daily torpor in mammals Torpor in a pocket-mouse ??

Summary to end nA large part of basal metabolism is to keep warm nInsulation u and its control nEndothermy u Facultative in insects nTorpor u energy saving

Reading … nPowerPoints on VLE or at nSchmidt-Nielsen, K (1997) Animal Physiology CUP